Guinea pig megakaryocytes have been isolated from bone marrow by combination of density equilibrium and velocity sedimentation techniques. The resulting cell suspension is approximately 20-30% megakaryocytes. P anned studies include: those on cell maturation with light microscopic, phase, Nomarski, electron microscopic techniques. Aspects on cell maturation, both nuclear and cytoplasmic, will be studied by micro- spectrophotometric, histochemical, and isotope incorporation techniques. Experimental design and microscopic techniques will be devised to demonstrate physiologic aspects of platlet formation both in vivo and in vitro. The effect of thrombopoietin on cell maturation will be compared to the normal condition.